Transient Stability Analysis of Grid-forming Converter Based on Virtual Synchronous Generator

This paper investigates transient angle stability of parallel system of two virtual synchronous generators (VSGs) and hybrid system comprised by VSG and diesel generator (DG). Compared to single grid-connected VSG, the paralleled VSGs system and hybrid system are facing larger challenges on stability aspect during serious faults. Although there exist several publications on transient stability, the transient instability mechanism is rather ambiguous, especially for paralleled systems. Besides, swing equations of paralleled VSGs system and paralleled DG-VSG system are established to discuss the dynamic characteristics of the system during short-circuit fault. The increase of power angle between DG and VSG is caused by the different angular accelerations, which deteriorates the transient angle stability of paralleled DG-VSG system. Thus, paralleled DG-VSG system is more prone to instability than paralleled VSGs system, which can be illustrated by power angle (P-δ) characteristic curves. Finally, simulation results validate the correctness of theoretical analyses.

Transient virtual inertia optimization strategy for virtual synchronous generator based on equilibrium point state assessment

The power transient stability analysis is one of the basis for determining the control strategy of power system security and stability. Considering the influence of power grid topology on the transient stability of power system, the transient stability evaluation model is constructed based on the graph attention neural network. The electrical components and their transient operation data are mapped to the graph data with the spatial topology characteristics of power system for model training, so as to improve the topological generalization performance of the model. The marginal contribution of input characteristics to the output of transient power angle stability evaluation model is quantitatively calculated based on Shapley additive explanation (SHAP), so as to improve the interpretability of data-driven method for transient power angle stability evaluation. The effectiveness of the proposed method is verified by the IEEE 39-bus system.

Transient stability analysis of a grid-connected type-4 wind turbine with grid-forming control during the fault

Virtual synchronous generators (VSGs) are prone to over-current and instability issues when grid faults occur. However, directly restraining the current reference in the control system of the VSGs will lead to a loss of connection between the inner current control loop and the outer power loops resulting in synchronization control failure and power angle instability. Besides, the VSGs are also susceptible to transient angle instability issues caused by losing stable operating point during grid faults. Therefore, achieving current limitations and maintaining the stable operation of the VSG is a worthwhile research topic. Hence, this article analyzes the adaptive current limiting characteristics of the VSG with a virtual impedance (VI) method and investigates the transient stability of the VSG under different fault clearance angles. It finds that the inherent growth of the VI during the current limiting process increases the risk of power angle instability. An improved control method is proposed in this article by adjusting the active power reference to account for the changes of the grid voltage and the VI, which is beneficial to enhance the power angle stability of VSGs during grid voltage drops. Besides, an estimation method of the grid voltage is adopted to deal with the instability issues caused by the uncertain grid voltage at the fault point. Then, the influence of grid impedance variation on the proposed method is analyzed, considering the voltage estimation errors, the power angle change, and the active power waste. Finally, the effectiveness of the proposed method is verified through simulation and experiment.

This paper presents the study of using a new hybrid simulator for transient stability assessment. This new hybrid simulator is developed to extend the traditional transient stability program to include detailed modelling of modern power electronics devices such as SVC, FACTS, etc. It possesses the capability of representing extensive power systems and modelling the particular devices in details using transient stability and electromagnetic transient models respectively. In the context of transient stability assessment, the hybrid simulator has the merit that it can analyze the influences of internal faults and malfunctions within devices such as FACTS and HVDC on systemwide stability. Case studies on three-generator nine-bus system are given to show the advantages of the hybrid simulator over traditional transient stability program.

With the development of science and technology, the cost of offshore wind power development and transmission has gradually decreased, making offshore wind power technology a research hotspot. The installed capacity has been increasing year by year. China has abundant offshore wind energy resources, which are much richer than those on land and are available for exploitation. Offshore wind speeds are stable and do not produce harmful substances. Therefore, studying the transient voltage stability of offshore wind power grid-connected systems to improve the utilization efficiency of offshore wind power and protect the ecological environment is critical. After large-scale wind power is connected to the grid, ensuring the stability of the system's transient voltage becomes crucial. This paper aims to deeply study the transient voltage stability of offshore wind farm grid-connected systems, providing theoretical support for the subsequent design and operation of offshore wind farms. A simulation model of a typical power system integrated with offshore wind farms was established using PSAT. Relevant transient voltage stability indices were calculated, and the impact of varying wind speeds on the transient voltage stability of the grid-connected system was investigated. Static Var Compensators (SVC) and Static Synchronous Compensators (STATCOM) were installed for comparative analysis. The results demonstrate that excessively low wind speeds degrade the system's transient stability, and the installation of SVC provides superior improvement in transient voltage stability compared to STATCOM.

A two-time step approach to transient stability modeling of detailed HVDC controls and L/R line dynamics is validated against a detailed electromagnetic transients model, and compared to the response-type HVDC model typically used in transient stability studies. The two-time step method combines the ability to include the detailed fast HVDC controls while still running the transient stability simulation of the large network at a 1/2 cycle time step, minimizing the impact to the overall computation time. To reduce complexity of the model, the detailed HVDC model assumes the AC commutating bus voltages to be unchanging during the internal fast time step loop of the HVDC link. Validation results show improved accuracy and numerical stability of the detailed HVDC transient stability model when compared to the typical response-type model, both in weak and strong test systems with effective short circuit ratios (ESCR) ranging from 2.0 to 5.0

Since the wide application of virtual synchronous generators (VSGs), the power grid faces great challenges in the safe and stable operation due to their limited thermal capacity and weak anti-disturbance ability. During transient period, for example, a fault occurs in the transmission line, the VSG may lose the transient angle stability and provoke the current hard limit. Even if the fault is cleared by tripping of line, it still faces the problem of instability and voltage dips. To address this problem, in this paper, the post-fault large-signal model of VSG is derived first via the travelling waves based fault information acquisition. Subsequently, with the effect of both active and reactive power loops taken into account, a two-stage simultaneous control scheme is proposed for improving the transient stability of VSG, while considering the current limitation during fault state and voltage support after fault clearance. This method is fulfilled by mode switching and an additional feedback control based on the fault signal. Finally, the effectiveness of the proposed method under both symmetrical and asymmetrical faults is verified. Moreover, the application of the proposed method in a multiple VSGs system is also verified. Besides, the robustness to parameter mismatch and the feasible operating region for the method are discussed.

This paper develops the transient stability enhancement of the inverter-based resources (IBRs) with the virtual synchronous generator (VSG) control employed. Firstly, based on the large-signal model, the instability phenomenon of the VSG control during voltage dips is fully analyzed. It is found that the frequency deviation Δω in the active power control loop (APCL) of VSG is always positive when the VSG is transiently unstable. Then, for guaranteeing stronger transient stability, an additional integral controller with the frequency deviation is added to the APCL in the VSG control. Moreover, a large-signal model of the proposed strategy is established. Then, the phase portraits with an integral controller in the APCL disabled/enabled are presented. It is seen that the improved control strategy has better transient stability under voltage dips. Finally, the simulation results verify the effectiveness of the proposed method.

Transient voltage stability assessment of renewable energy grid based on residual SDE-Net

Attributed to the ability of inertia provision and good regulation performance, the virtual synchronous generator (VSG) has been proven as a promising solution to the problems introduced by using converter interfaced generation. Although the small-signal stability control of the VSG has been widely analyzed, the transient characteristics still needs more study, which belongs to a large-signal problem. In this article, the transient angle stability control of the VSG is investigated. Two possible VSG emulation methods, i.e., the power-emulation and the torque-emulation, are compared from the perspective of transient characteristics. Then, the transient of the VSG internal voltage and its impact on the transient angle stability are quantitatively studied in details. Thereafter, an enhanced VSG controller is proposed, where a large-signal analysis is also presented to evaluate its influence on the acceleration and deceleration areas. Finally, the analysis and effectiveness of the enhanced VSG method are validated by the experimental results.

This paper examines a completely novel method for improving the transient stability of multi virtual synchronous generator (VSG) grids. In this method, it has been tried to control the oscillation of VSGs with respect to the center of inertia frequency (COI-frequency) of the grid, during short-circuit. It is also well demonstrated here that the proposed method is able to prevent the increase in irreversible distance angle of VSGs relative to each other, which is the main factor in preventing VSGs reaching to instability point. The method can enhance both the synchronizing and damping terms efficiently. Detailed analyzing of the direct Lyapunov method illustrates the non-linear proof of the method. Also, non-linear algebraic equations for a two-VSG system have been derived to prove the possibility of corroborating simulation results. Equal area criterion is also used to clarify this method further. One interesting feature of the method is that there is no conflict between the requirements of the low voltage ride-through and transient stability improvement.

With widespread application of voltage source converter (VSC) as a key energy-conversion power electronic device by using the phase-locked loop (PLL) technique for synchronization, the system dynamics has become much complicated. In this paper, the nonlinear dynamics and transient stability of the PLL-based VSC system are investigated, within a unified framework of the (normalized) generalized swing equation. It is found that there are three different types of bifurcation, including the generalized saddle-node, Hopf, and homoclinic bifurcations. Within the coexistence parameter region, the basin boundary of the stable equilibrium point shows either a closed-loop or a fish-like pattern. With the help of the equal area criterion (EAC), the transient stabilities under different transient disturbances including short circuit, voltage dip, and power rise are analyzed. Because the equivalent damping of the VSC is state-dependent, the theoretical results based on the EAC are examined. Furthermore, based on the analytical results from the generalized swing equation, the principle for all major transient stability enhancement methods is uncovered. All these findings are well verified by extensive electromagnetic transient simulations. Therefore, the generalized swing equation provides a deeper physical insight and plays a crucial role in transient stability problems in power-electronic-dominated power systems, similar to the swing equation in traditional power systems.

Chapter 18 - Power System Stability